Complete the table and draw the fourth stage of the pattern. The pattern is formed by triangles made of matchsticks.

The sequence is 3, 5, 7. The difference between consecutive terms is $5-3=2$ and $7-5=2$. This is an arithmetic sequence where each term increases by 2.

For Pattern 4: $7 + 2 = 9$
For Pattern 10: Using the rule $1 + (2 \times \text{Pattern number})$, for Pattern 10, it's $1 + (2 \times 10) = 1 + 20 = 21$.

For Pattern 4: $1+2+2+2+2$
For Pattern 10: $1 + (2 \text{ repeated 10 times})$

For Pattern 4: $1+(2\times4)$
For Pattern 10: $1+(2\times10)$

Here is the completed table:

The general rule for the number of matchsticks ($M$) for a given pattern number ($N$) is:

Here, the number of triangles is $N = 20$.

The number of matchsticks needed for 20 triangles is $\boxed{41}$.